Known in the art is a method of measuring optical losses in fiber light guides in reflected light (ref. USSR Application No. 4204727, with a decision to issue an Inventor's Certificate passed on May 30, 1988), comprising a probing radiation pulse into a test light guide through a blank light guide, the length of which is equal to half the length of the established mode distribution for the given type of light guide, measuring the energy N.sub.1 of a pulse reflected from the output end of the blank light guide, measuring the energy N.sub.2 of a pulse reflected from the output end of the blank light guide and the input end of the test light guide located at a minimum distance from each other, measuring the energy N.sub.3 of a pulse reflected from the output end of the test light guide and determining optical losses .mu. per unit length of the test light guide.
This prior art method is disadvantageous in that it cannot be used for determining optical losses at the radiation input and output of the test light guide, i.e., at the output end of the blank light guide and at the input and output ends of the test light guide, and also in the air gap between the joined light guides because of the non-parallel alignment of their ends and the non-coaxial alignment of their cores.
Also known in the art is a method of determining optical losses in joints of light guides according to scattered radiation propagating in the backward direction (ref. Shiketants, D., "Theory of Measurement by Backward Scattering Method in Light Guides" in Zarubezhnaya Radioelektronika, 1981, No. 6, pp.87-94), comprising sending radiation into the light guides being joined, measuring the intensity of radiation scattered in the light guides upstream and downstream of the joint, and determining optical losses in the joint of the light guides.
As the closest prior art of the present method in technical essence, this method has been selected as the prototype.
A drawback of the prior art method is the low intensity of the scattered radiation, which limits the maximum possible distance in the light guide line to the test joint.
The need exists, however, and its importance rises with an increase in the length of trunk light guide communication cables the length of which presently reaches 30 to 40 kilometers or more, and which are accessed by measuring devices from one side only.